For a good understanding of the invention, the rest of the description relates to the precise example of an equipment item for which the electrical quantity which varies as a function of temperature is the gain or the phase of the equipment item and where the digital control is a control voltage but it is obvious that the invention is not limited to this electrical quantity and to this type of control.
As represented in the example of FIG. 1, the compensation of the variations of an electrical quantity of a radiofrequency equipment item 10 as a function of temperature, for example the variations of the gain or of the phase of a radiofrequency chain comprising, linked in series, amplifiers 11 and attenuators 12 that can be controlled, for example in voltage, is conventionally produced from a single analogue control Vc applied at the input of the equipment item. For this, from a setpoint, for example a gain setpoint, imposed by a user, a control device 16 generates an analogue control 17 which is applied to the equipment item 10 via an electronic circuit based on resistors and thermistors 18, the thermistors ensuring the temperature compensation of the equipment item. The compensation of the variations, for example of the gain, of an equipment item by analogue control therefore entails adding wiring for resistors and thermistors directly to the equipment item 10, which increases the weight, the bulk and the cost of the equipment item. Furthermore, the value of these components has to be optimized finely and requires the use of expensive methods involving temperature cycles to determine the values of the components, check the performance levels and repeat these temperature cycles and these checks until the objective concerning the desired performance levels, such as the temperature gain stability for example, is reached.
The compensation of the variations of the gain, or of the phase, or of any other electrical parameter or quantity, of a radiofrequency equipment item 10 can also be performed from a digital control, which makes it possible to improve the competitiveness of the equipment item by virtue of the cost and weight reductions which result therefrom and to reduce the equipment item setting time. However, contrary to an analogue control which corresponds to the application of a continuous value, for example voltage value, at the input of the equipment item and to a continuous variation of the electrical quantity of the equipment item, for example of the gain or of the phase of the equipment item, a digital control consists of a succession of discontinuous digital states, called digital control words, stored in look-up tables at different addresses of a memory. Each stored digital control word corresponds to a digital control value to be applied to the equipment item for a gain setpoint value imposed by the user and for a temperature value measured on the equipment item. However, on each change of state, these discontinuous states induce, on the curve of variation of the electrical quantity, for example of the gain, of the equipment item, transient spurious fluctuations in amplitude and also in phase, as well as other possible secondary effects as a function of the nature of the physical quantities controlled, which can be very rapid and which, if they are not controlled, can disturb the telecommunications traffic bit rate and quality and induce a risk of desynchronization of the telecommunications receivers in the communication chain including the equipment item. To resolve these problems, it is conventional practice to increase the number of states stored in the memory and the associated memory space. However, this solution is limited by the capacity of the memory available, entails the use of analogue/digital convertors operating on a number of digital bits that is all the higher as the number of states increases, which does not make it possible to address the absence of traffic interruption in case of a change of the value of the electrical quantity, for example of the gain or of the phase, by remote control.